OK, so the only way to actuate it is to press on the center piece, it doesn't latch, I would guess either
a) its made for removing something, rather than installing. If that something were moving in and out, that would explain the need for the rubber bump-stop for the arms or b) you retract it by hand, and place something with a hole in the middle (like a washer, or maybe it projects out more than that, like a hollow cylinder with a ridge on the bottom) on the inside of the 4 nubs (its flattened on the arms as though to accomodate something) and then when you go to install it onto something, that something presses on the center or on the washer you're installing, causing it to release. maybe that action takes a bit of force or is automatic, and hence the rubber bumper.
Dave